The invention concerns a procedure for generating a tomographic image of a three-dimensional object in a sectional plane. It extends furthermore to improved tomographic equipment to implement this procedure.
Experts are presently well versed in tomographic technology of which the object is to provide an image of a specific sectional plane of an object. This technology is applied essentially in the medical domain to observe tissues internally at particular levels. Compared to the simple methods of radiology, tomography offers more significant images of the zone under observation with a concurrent reduction of the defects due to the other zones. This technique consists in irradiating the object at different angles of incidence, in generating for each incident angle a corresponding image signal, and in processing these various signals to synthesize the image. Conventionally this processing comprises a photographic integration in order to reproduce a sharp picture of those points which are located in the sectional plane of the various incident beams. More recently, for the last fifteen years or so, particular procedures have been developed which electrically integrate electrical-image signal (illustratively the radiological tomoscopic procedure described in the French Pat. No. 1,591,807 of May 4, 1970 [SEREL].
Whether these procedures integrate photographically or electrically, they incur the drawback of image complexity and poor densitometric resolution, whereby the images are difficult to interpret. Illustratively, while this image does include a sharp representation of the points of the sectional plane of interest, it also contains the superposition of more or less blurred curves from the other zones by the very principle of integration adding all the collected data.
Some apparatus have been designed to eliminate this drawback. The equipment described in the French Pat. No. 2,415,400 of Jan. 10, 1979 (PHILIPS) comprises several radiation sources whereby several images can be collected by a fixed receiver. These images are processed by an optical system, then by computer means, in order to reconstitute a synthesized image at a particular level. While it seems that blurring can be reduced by this method, it cannot be rigorously eliminated. Moreover, this technique is inapplicable to conventional tomographs and requires the construction of a special tomograph with several sources and with a specific optic system, explaining why presently this technique still lacks implementation.
Another method described in the French Pat. No. 2,469,739 (PHILIPS) of Nov. 5, 1980 comprises taking two photographs on one tomograph of which the source is subjected to two sweeps of different geometries (for instance circular and linear). These two photographs then are analyzed by two vide cameras in order to eliminate the shadows from the different geometries corresponding to points which are external to the sectional plane. The implementation of this method is lengthy and complex because requiring taking and developing two photographs and subsequent processing by a separate apparatus.
With regard to other types of apparatus, it is known how to process image signals fitted to the mandates of each particular application. Thus U.S. Pat. No. 4,437,161 (ANDERSON) describes an angiographic apparatus wherein, on one hand, several time-shifted images of a patient's organ are obtained following injection of contrast media from one fixed and single radiation source, and on the other hand, a differential image is processed by subtracting from each image a stored mask consisting of one of the suitably selected images. In this patent's apparatus, this differential image is compared pixel by pixel to two fixed levels in order to ascertain the applicability of the mask being used and, where called for, to store the significant received images. The comparison is solely to eliminate the spurious images (patient's motions or the as yet untriggered action of the contrast medium), but in no event do they cause a modification in the received images. The significant images then are stored and processed by subtraction in conventional manner in angiography to as to obtain a synthesized image.
However such specific processing does not resolve the problem of tomographic blurs and in no way teaches a way to a solution to the expert.
An object of the present invention is to resolve this problem by a very easily implemented and economical procedure applicable to conventional tomographs.
The object of the invention therefore is to use a conventional type tomograph to provide a tomographic image strictly free of blurred traces due to points outside the sectional plane.
Another object of the invention is to apply this procedure to the most widespread type of tomography which includes a movable source of radiation, a fixed support for the object to be examined, and oppositely, a transducer moving synchronously with the source.
In some tomographs, the various positions of the radiation source(s) during the tomographic sweep correspond to very different beam incidences and distances with respect to the photosensitive surface of the transducer. These variations in distance and angle of incidence cause substantial image perturbations because of the more than trivial radiation-attentuation factors which then intervene and which are spurious with respect to the object being examined. No known device can be rid of these defects.
Another significant object of the invention is to eliminate the effect of the variation in radiation intensities on the image due to the variations in source distance and angle of incidence with respect to the transducer.